Time- and Energy-Resolved Emission Spectroscopy Using a Step-Scan FT Spectrometer Combined with Correlational Analysis Techniques

The decay of photoluminescence emission signals on the time scale from microseconds to seconds can be measured with superior dynamics and signal-to-noise ratio by a new method which makes use of correlation analysis with pseudorandom binary sequences. Instead of excitation of the sample by one short laser pulse for each decay cycle (which results in low average emission intensity), the sample is pumped with a continuous-wave laser modulated by a pseudorandom sequence with δ-function-like autocorrelation properties. Therefore, on the time average half of the exciting laser power pumps the sample, and the resulting high emission intensity allows recording of photoluminescence decays over as many as four orders of magnitude within measurement times of 10 min. When this technique is combined with a step-scan Fourier transform spectrometer, both time and energy resolution can be obtained simultaneously. For each interferometer step, the sample response to the excitation sequence is recorded, later autocorrelated digitally, and combined to interferograms for each time step, and finally Fourier transformed. With this technique, time-resolved high-sensitivity spectra can be recorded in the NIR, where only detectors with relatively poor detectivity (D) are available, and in the visible spectral range. Preliminary results obtained from relatively slow emission processes at defects in semiconductors are presented, which show decay constants on microsecond to millisecond scales.